Conventionally employed magnetic recording media, such as audio tapes or discs, video tapes or discs, computer data tapes, and the like, comprise a non-magnetic support having provided thereon a magnetic layer comprising a binder having dispersed therein a ferromagnetic powder, e.g., a powder of .gamma.-Fe.sub.2 O.sub.3, Co-doped iron oxide, chromium dioxide, ferromagnetic alloys, etc. With the recent demands for these magnetic recording media to have an increased recording density and an increased S/N ratio, there is a tendency to use finer particles of the magnetic substance. However, as the magnetic powder becomes finer, abrasion resistance of the powder is reduced, resulting in deterioration of running durability of the resulting magnetic recording media. This tendency is particularly conspicuous when in using ferromagnetic alloy fine powders having low hardness.
In order to prevent deterioration in running durability, it has been proposed to add a granular abrasive, such as Al.sub.2 O.sub.3, SiC, Cr.sub.2 O.sub.3, etc., to a magnetic layer. The abrasives to be used for this purpose include particles of hard substances, such as chromium oxide, fused alumina, silicon carbide, corundum, diamond, emery (consisting mainly of corundum and magnetite), siliceous substances, silicon nitride, boron nitride, tungsten carbide, titanium oxide, etc. These abrasives may be used either individually or in combinations of two or more thereof.
The inventors previously researched into various combinations of the above-described abrasives and, as a result, found that running durability and electromagnetic characteristics of a magnetic recording medium can be improved markedly by using a magnetic layer containing acicular Cr.sub.2 O.sub.3 having a length in the long axis of not more than 1.0 .mu.m, a length in the short axis of not more than 0.1 .mu.m, and an acicular ratio of from 5 to 20 in combination with an inorganic powder of at least one member selected from .alpha.-Al.sub.2 O.sub.3, SiC, and TiC, a ratio of the acicular Cr.sub.2 O.sub.3 to the inorganic powder being greater 2/8, and the total amount of said acicular Cr.sub.2 O.sub.3 and inorganic powder ranging from 1 to 20% by weight based on a magnetic powder, as disclosed in Japanese Patent Application (OPI) No. 109227/87(the term "OPI" as used herein means a "published unexamined Japanese Patent Application).
Incorporation of a large quantity of the aforesaid abrasives aiming at improvement on running durability is attended by deterioration in magnetic orientation of the medium, filling degree or dispersibility of a magnetic powder, which would lead to reduction of electromagnetic characteristics due to deteriorated surface properties.
Surface properties of the magnetic layer may be improved by reduction of the used amount of the abrasive, and whereby the electromagnetic characteristics is improved. But the magnetic layer have a poor running properties due to causes such as an increased coefficient of friction, and an adhesion of tapes upon running.
The characteristics relating to a magnetic layer such as scractch resistance or running durability of a magnetic layer are varied depending on conditions between the surface of a magnetic layer and the surface of a magnetic head. Therefore, if an abrasive is incorporated into a magnetic layer for the purpose of improving various characteristics, it is only those abrasive particles exposed on the surface of the magnetic layer that make a contribution to such improvements, while those particles buried inside the magnetic layer produce no effects. From this viewpoint, the surface of the abrasive particles take an important role in magnetic recording media. However, the improvements attained by the conventional abrasives are so far confined within narrow ranges of known characteristics, and striking improvements have not been expected.